Add object constraint follow path
Because Follow Path will take into account the location of the object relatively to the world origin, so when you move the curve AND the planet object, the planet object moves twice as it is attached to the curve, and as it changes its location relatively to the world origin.
To fix that, you can either:

Duplicate the curve and planet and only move the curve, the planet will move along, or
Move both, then select each planet and press AltG (to bring back to world origin)